Common causation and offset effects in human visual-inertial heading direction integration

Influence of Tactile Flow on Visual Heading Perception

The integration of information from different sensory modalities is crucial for successful navigation through an environment. Among others, self-motion induces distinct optic flow patterns on the retina, vestibular signals and tactile flow, which contribute to determine traveled distance (path integration) or movement direction (heading). While the processing of combined visual-vestibular information is subject to a growing body of literature, the processing of visuo-tactile signals in the context of self-motion has received comparatively little attention. Here, we investigated whether visual heading perception is influenced by behaviorally irrelevant tactile flow. In the visual modality, we simulated an observer's self-motion across a horizontal ground plane (optic flow). Tactile self-motion stimuli were delivered by air flow from head-mounted nozzles (tactile flow). In blocks of trials, we presented only visual or tactile stimuli and subjects had to report their perceived heading. In another block of trials, tactile and visual stimuli were presented simultaneously, with the tactile flow within ±40° of the visual heading (bimodal condition). Here, importantly, participants had to report their perceived visual heading. Perceived self-motion direction in all conditions revealed a centripetal bias, i.e., heading directions were perceived as compressed toward straight ahead. In the bimodal condition, we found a small but systematic influence of task-irrelevant tactile flow on visually perceived headings as function of their directional offset. We conclude that tactile flow is more tightly linked to self-motion perception than previously thought.

Effect of timing delay between visual and vestibular stimuli on heading perception

Heading direction is perceived based on visual and inertial cues. The current study examined the effect of their relative timing on the ability of offset visual headings to influence inertial perception. Seven healthy human subjects experienced 2 s of translation along a heading of 0°, ±35°, ±70°, ±105°, or ±140°. These inertial headings were paired with 2-s duration visual headings that were presented at relative offsets of 0°, ±30°, ±60°, ±90°, or ±120°. The visual stimuli were also presented at 17 temporal delays ranging from -500 ms (visual lead) to 2,000 ms (visual delay) relative to the inertial stimulus. After each stimulus, subjects reported the direction of the inertial stimulus using a dial. The bias of the inertial heading toward the visual heading was robust at ±250 ms when examined across subjects during this period: 8.0° ± 0.5° with a 30° offset, 12.2° ± 0.5° with a 60° offset, 11.7° ± 0.6° with a 90° offset, and 9.8° ± 0.7° with a 120° offset (mean bias toward visual ± SE). The mean bias was much diminished with temporal misalignments of ±500 ms, and there was no longer any visual influence on the inertial heading when the visual stimulus was delayed by 1,000 ms or more. Although the amount of bias varied between subjects, the effect of delay was similar.NEW & NOTEWORTHY The effect of timing on visual-inertial integration on heading perception has not been previously examined. This study finds that visual direction influence inertial heading perception when timing differences are within 250 ms. This suggests visual-inertial stimuli can be integrated over a wider range than reported for visual-auditory integration and may be due to the unique nature of inertial sensation, which can only sense acceleration while the visual system senses position but encodes velocity.